B&K Vocoder

In this new year I will work on a new project, the B&K Vocoder. But what is a Vocoder?
If you do not know, but you want to know :), please check this link.

To create this Vocoder will be a big project that consists of two original B&K Band Pass Filter Sets Types 1615 (see photo) in combination with a 32×32 OSC controlled audio matrix – the CompLex.  The 3rd octave B&K-filters are made in the 1970’s and make use of real big coils. This makes the sound of these bandpass filters very unique. Since we have a few of these filters and we can use the OSC-controlled audio matrix to change the routing, the whole device will be an interesting composing tool for the department of Sonology. See also the block schematic below.

Two identical Bruel & Kjear Bandpass filter 1615:

First block schematic of the B&K Vocoder:

Short description of this block schematic::
The speech input signal will be split into 20 different frequency parts – the actual spectral characteristic. These separate split frequency parts will be converted to an envelope by a so called Amplitude Demodulator (or amplitude follower). These 20 x AMD’s will ‘follow or rectify’ the amplitude of the audio signal and create a low frequent control signal, that is a representative of the actual amplitude. The 20 different ‘envelopes’ will be connected to an audio-matrix (the extended CompLex ;-).

On the second input a miscellaneous signal (music, or whatever is available) can be connected. This second signal is equally split into 20 single frequency’s – exactly like the speech input.  All separate split frequency parts of the second signal are now linked to 20 x AD633 ‘multiplier chips’.  If the audio matrix is set to In1 = Out1, In2 = Out2,  …, In20 = Out20, the 20 different amplitudes of the ‘speech’ input signal are now multiplied (modulated) with the same split frequencies of the second input.

The interesting part of this design is that we can change the audio-matrix on the fly with OpenSoundControl (OSC) or with other analogue signals and so changing the route of the control voltages. For example the envelope of the 1kHz band of the speech can now be connected to the 20Hz band  of the second input. This will really create very interesting (musical) results.

For now I will start with the explanation and design of the Amplitude Demodulators.